Glass for magnetic recording media substrates, magnetic recording media substrates, magnetic recording media and method for preparation thereof

Abstract

A glass for a magnetic recording medium substrate permitting the realization of a magnetic recording medium substrate affording good chemical durability and having an extremely flat surface, a magnetic recording medium substrate comprised of this glass, a magnetic recording medium equipped with this substrate, and methods of manufacturing the same. The glass is an oxide glass not including As or F.

The invention claimed is: 
     
       1. A method for manufacturing a glass for a magnetic recording medium substrate comprised of an oxide glass, characterized by:
 preparing a glass starting material to which Sn and Ce are added, comprising, as converted based on the oxide, denoted as molar percentages: 
 SiO 2  60 to 75 percent, 
 Al 2 O 3  1 to 15 percent, 
 Li 2 O 5 to 10 percent, 
 Na 2 O 8 to 15 percent, 
 P 2 O 5  0 percent, and 
 K 2 O 0 to 5 percent, 
 (wherein one or more of MgO, CaO, SrO and BaO is present and the total amount of MgO, CaO, SrO and BaO is in a range of 0.1 to 5% and a total content of Li 2 O, Na 2 O, and K 2 O is 25 percent or lower); 
 so as to permit obtaining a glass containing a total quantity of Sn oxide and Ce oxide of 0.5 mass percent to 1.5 mass percent based on the total amount of the glass components, wherein the ratio of the content of Sn oxide to the total content Sn oxide and Ce oxide (content of Sn oxide/(content of Sn oxide+content of Ce oxide)) is 0.45 to 0.85, having an Sb oxide content of 0 to 0.1 percent, and comprising no As or F; 
 melting the glass starting material; 
 clarifying the resulting glass melt; and 
 molding the resulting glass melt, 
 wherein the molded glass contains 2 pieces/kg or less of residual bubbles. 
 
     
     
       2. The method for manufacturing a glass for a magnetic recording medium substrate according to  claim 1 , wherein the glass melt obtained by preparing and melting the glass starting material is maintained at 1,400 to 1,600° C., the temperature is decreased, the glass melt is maintained at 1,200 to 1,400° C., and the glass melt is molded. 
     
     
       3. The method for manufacturing a glass for a magnetic recording medium substrate according to  claim 1 , wherein a viscosity of the glass melt at 1,400° C. is 10 3  dPa·s or lower. 
     
     
       4. The method for manufacturing a glass for a magnetic recording medium substrate according to  claim 1 , wherein the glass melt is made to flow out to obtain glass melt gobs and the glass gobs are press molded. 
     
     
       5. The method for manufacturing a glass for a magnetic recording medium substrate according to  claim 1 , wherein the glass melt is molded into a sheet of glass by the float method. 
     
     
       6. The method for manufacturing a glass for a magnetic recording medium substrate according to  claim 1 , wherein the glass melt is molded into a sheet of glass by overflow down draw molding. 
     
     
       7. A method for manufacturing a magnetic recording medium substrate, comprising:
 a step of preparing a glass by the method described in  claim 1 ; 
 a step of mirror-surface polishing the glass; and 
 following mirror-surface polishing, a cleaning step of cleaning with an acid and cleaning with an alkali. 
 
     
     
       8. A method for manufacturing the magnetic recording medium substrate according to  claim 7 , comprising a step of subjecting the glass to a chemical strengthening treatment between the mirror-surface polishing step and the cleaning step. 
     
     
       9. The method for manufacturing a glass for a magnetic recording medium substrate according to  claim 1 , wherein the glass starting material comprises 0.1 to 5 percent of MgO and CaO in total, 0 to 1 percent of ZnO, and 0.1 to 5 molar percent of ZrO 2 , TiO 2 , La 2 O 3 , Nb 2 O 5 , Ta 2 O 5 , and HfO 2  in total,
 wherein a ratio of the ZrO 2  content to the total content of ZrO 2 , TiO 2 , La 2 O 3 , Nb 2 O 5 , Ta 2 O 5 , and HfO 2  (ZrO 2 /(ZrO 2 +TiO 2 +La 2 O 3 ,+Nb 2 O 5 +Ta 2 O 5 +HfO 2 )) is 0.9 to 1. 
 
     
     
       10. A method for manufacturing a magnetic recording medium substrate, comprising:
 a step of preparing a glass by the method described in  claim 4 ; 
 a step of mirror-surface polishing the glass; and 
 following mirror-surface polishing, a cleaning step of cleaning with an acid and cleaning with an alkali. 
 
     
     
       11. A method for manufacturing a magnetic recording medium substrate, comprising:
 a step of preparing a glass by the method described in  claim 9 ; 
 a step of mirror-surface polishing the glass; and 
 following mirror-surface polishing, a cleaning step of cleaning with an acid and cleaning with an alkali. 
 
     
     
       12. A method for manufacturing a magnetic recording medium substrate, comprising:
 a step of preparing a glass by the method described in  claim 1 ; and 
 a step of mirror-surface polishing the glass. 
 
     
     
       13. A method for manufacturing a magnetic recording medium substrate, comprising:
 a step of preparing a glass by the method described in  claim 4 ; 
 a step of mirror-surface polishing the glass. 
 
     
     
       14. A method for manufacturing a magnetic recording medium substrate, comprising:
 a step of preparing a glass by the method described in  claim 5 ; and 
 a step of mirror-surface polishing the glass. 
 
     
     
       15. A method for manufacturing a magnetic recording medium substrate, comprising:
 a step of preparing a glass by the method described in  claim 6 ; and 
 a step of mirror-surface polishing the glass. 
 
     
     
       16. A method for manufacturing a magnetic recording medium substrate, comprising:
 a step of preparing a glass by the method described in  claim 9 ; and 
 a step of mirror-surface polishing the glass. 
 
     
     
       17. A method for manufacturing a glass for a magnetic recording medium substrate comprised of an oxide glass, characterized by:
 preparing a glass starting material to which Sn and Ce are added, comprising, as converted based on the oxide, denoted as molar percentages: 
 SiO 2  60 to 75 percent, 
 Al 2 O 3  1 to 15 percent, 
 Li 2 O 5 to 10 percent, 
 Na 2 O 8 to 15 percent, 
 P 2 O 5  0 percent, and 
 K 2 O 0 to 5 percent, 
 (wherein one or more of MgO, CaO, SrO and BaO is present and the total amount of MgO, CaO, SrO and BaO is in a range of 0.1 to 5% and a total content of Li 2 O, Na 2 O, and K 2 O is 25 percent or lower); 
 so as to permit obtaining a glass containing a total quantity of Sn oxide and Ce oxide of 0.5 mass percent to 1.5 mass percent based on the total amount of the glass components, wherein the ratio of the content of Sn oxide to the total content Sn oxide and Ce oxide (content of Sn oxide/(content of Sn oxide+content of Ce oxide)) is 0.45 to 0.85, having an Sb oxide content of 0 to 0.1 percent, and comprising no As or F; 
 melting the glass starting material; 
 clarifying the resulting glass melt; 
 molding the resulting glass melt; 
 further processing the molded glass, and 
 forming a magnetic layer on the further processed glass. 
 
     
     
       18. The method for manufacturing a magnetic recording medium according to  claim 17 , wherein the glass melt obtained by preparing and melting the glass starting material is maintained at 1,400 to 1,600° C., the temperature is decreased, the glass melt is maintained at 1,200 to 1,400° C., and the glass melt is molded. 
     
     
       19. The method for manufacturing a magnetic recording medium according to  claim 17 , wherein a viscosity of the glass melt at 1,400° C. is 10 3  dPa·s or lower. 
     
     
       20. The method for manufacturing a magnetic recording medium according to  claim 17 ,
 wherein the glass starting material comprises 0.1 to 5 percent of MgO and CaO in total, 0 to 1 percent of ZnO, and 0.1 to 5 molar percent of ZrO 2 , TiO 2 , La 2 O 3 , Nb 2 O 5 , Ta 2 O 5 , and HfO 2  in total, and 
 wherein a ratio of the ZrO 2  content to the total content of ZrO 2 , TiO 2 , La 2 O 3 , Nb 2 O 5 , Ta 2 O 5 , and HfO 2  (ZrO 2 /(ZrO 2 +TiO 2 +La 2 O 3 ,+Nb 2 O 5 +Ta 2 O 5 +HfO 2 )) is 0.9 to 1.